I. Field of the Invention
The invention relates to the field of turbojets and particularly turbofan engines with the fan fixed to a drive shaft that is supported by a first bearing and a second bearing.
This type of turbojet comprises, in order from the upstream side to the downstream side along the gas flow direction, a fan, one or several compressor stages, one or several turbine stages and a gas exhaust nozzle. The fan comprises a rotor provided with blades around its periphery which, when rotated, entrain air in the turbojet. The fan rotor is supported by the engine low pressure compressor shaft. It is centred on the centre line of the turbojet by a first bearing that is on the upstream side of a second bearing connected to the fixed structure, particularly the intermediate case.
In the remainder of the description, this shaft is referred to simply as the compressor shaft, since the fan is fixed to the compressor shaft, which is the low pressure rotor shaft in a twin shaft engine.
The first bearing is supported by a support part forming an enclosure around the compressor shaft, oriented towards the downstream side of the first bearing and fixed to a fixed structure of the turbojet. The second bearing is supported by a support part also fixed to a fixed structure of the turbojet.
II. Description of Related Art
A fan blade can be lost accidentally. This creates a large unbalanced mass on the compressor shaft which introduces loads and vibrations on the bearings, transmitted through their support parts to fixed structures of the turbojet, that can consequently be damaged.
The risk of excessive damage to the turbojet can be prevented by oversizing the structure, or by providing a decoupling system for the first bearing, as described in patent FR 2 752 024. The support part of the first bearing is fixed to the turbojet structure by so-called shear screws, in which there is a weakened portion at which the screws will break if excessive forces are applied. Thus, when an unbalanced mass appears on the compressor shaft, the forces induced on the first bearing are transmitted to the shear screws that break, decoupling the support part from the first bearing, and therefore the first bearing from the turbojet structure. Forces created by the unbalanced mass are no longer transmitted to the fixed structure of the turbojet through this support part.
However, the fan continues to rotate. The unbalanced mass causes bending of the compressor shaft that can cause displacement between the inner and outer bearing rings in the second bearing. Furthermore, bending of the shaft introduces a lever effect that applies stresses to the bearing rollers or balls. The result is possibly breakage of the second bearing.
Patent FR 2831624, also issued by the same applicant, describes a means in which the second bearing is mounted with a radial clearance in a reaming in the shroud support. The support of the second bearing is clamped by screws parallel to the shaft and that will fuse in shear. Consequently, the second bearing can be supported in the shroud support. This reduces the risk of friction.